This invention relates to improvements in video recorder-playback machines, and more specfically, to a lens assembly for a video recorder-playback machine, including improved means for adjusting the focus of an optical signal upon a video information disc.
Video recorder-playback machines in general are known in the art and include appropriate means for recording and/or playing back a video information signal utilizing a selected medium for storage of the signal. For example, in one type of machine, the video signal is recorded magnetically for storage and/or retrieval upon a length of so-called video tape. In another type of system, the video signal is recorded upon an information disc for retrieval by means of a stylus, in a manner generally analogous to that of sound reproduction from phonographic records. In still another type of system, the video signal is used to frequency modulate a beam of amplified light, such as a laser beam, with sufficient power for physically altering the surface characteristics of a light sensitive coating on a video information disc and thereby record the signal upon the disc. During playback, a lower power light beam is reflected from the disc and the resulting signal is demodulated to reproduced the recorded signal. In all of these types of systems, the video information signal can be combined with an appropriate audio signal for either recording or playback purposes to yield a composite audio-video signal of the type commonly used for television transmissions and the like. For convenience, however, the signal will be referred to herein as a video information signal.
Video recorder-playback machines utilizing amplified light beams for recording and playback purposes offer significant advantages in that all physical contact of the recording and playback elements with the storage medium, namely, the information disc, is avoided. This prevents wear and deterioration of the machine elements and the disc, resulting in a high quality stored video signal which can be played back repeatably over a long period of time with unimpaired video resolution.
In video recorder-playback machines using amplified light beams for signal recording and playback, the recording and playback optical beams are focused upon the video information disc by a focusing lens which traverses the span of the disc in a radial direction simultaneously with relatively high speed rotation of the disc. In this manner, a spiral pattern of closely spaced tracks is formed on the disc to represent the video information. For maximum storage capacity for each disc, together with maximum signal resolution, it is desirable to form the information tracks on the disc to have a narrow width on the order of about 0.5 microns, and a narrow center-to-center spacing between radially adjacent tracks in the spiraling pattern on the order of about 1.5 microns. Accordingly, for high quality signal resolution without cross talk between adjacent tracks during recording and/or playback operation, it is imperative that the amplified light beams are accurately and precisely focused at all times upon the video information disc. However, video information discs typically exhibit surface imperfections resulting from scratching, pitting, warping, and the like. These surface imperfections create relatively slight variations during disc rotation in the distance between the focusing lens and the disc to impair precise and consistent focusing of the light beams on the disc.
A variety of systems have been proposed for positionally adjusting the focusing lens in response to surface imperfections in the disc in an effort to maintain a precise focus of the optical beams upon the disc. Some of these systems comprise servo mechanisms wherein the focusing lens is positionally adjusted by means of mechanical linkage arrangements. See, for example, U.S. Pat. No. 3,829,622. However, these mechanical systems are subject to vibration and wear during operation resulting in a decrease in system accuracy over time. Other systems have proposed fluid or hydrodynamic pressure responsive devices for adjusting the position of the focusing lens, such as those shown and described in U.S. Pat. Nos. 3,894,180; 3,914,541; 3,947,888; and 4,006,294. However, these systems are relatively complex in design and are not easily packaged into an inexpensive and compact unit for use with record or playback machines on a commerical scale. Still other systems have suggested the use of electromagnetic devices for controlling the position of a focusing lens supported for movement within an air bearing. See, for example, U.S. Pat. No. 3,530,258. However, these systems also have been relatively complex in design and operation making them difficult to package in a compact and inexpensive unit for large scale commercial production.
The present invention overcomes the problems and disadvantages of the prior art by providing an improved lens assembly for a video recorder-playback machine. The improved lens assembly includes a simplified, compact, and relatively inexpensive arrangement for electromagnetic adjustment of the position of a focusing lens carried within an air bearing.